It is essential that the two gripper heads do not collide with each other when they come closest to each other in the center of the shed for the transfer of the weft thread. Thus, proper guiding of the gripper heads is essential. Two basic possibilities are available for solving this problem. In the majority of tape gripper head looms very light flexible synthetic material tapes are used for carrying a gripper head at one end of the tapes which are guided over the entire weaving width in so-called guide teeth. The guide teeth are so arranged that during the travel of the gripper these teeth grip from below through the warp threads of the lower shed. The guide teeth are rigidly connected with the reed beam carrying a plurality of reeds to form a slay. When the slay is in the beat up position, the guide teeth dip below the warp threads of the lower shed. In connection with the just described structures, reference is made to European Patent Publications 0,204,274 (Pezzoli), published Dec. 10, 1986, and 0,402,285 (Riolet), published Dec. 12, 1980.
The type of gripper guide as described above has the disadvantage that the guide teeth dip below the warp threads of the lower shed only relatively shortly before the slay comes into the beat up position, whereby the lower shed warp threads do not tense-up fast enough before the beat-up. Thus, there is the danger that irregularities occur in the finished fabric in those positions corresponding to the point where the guide teeth dip under the lower shed warp threads since the latter are insufficiently tense at the time of beat-up. Such irregularities may appear in the form of stripes or the like. Another disadvantage of the just described construction is seen in that a relatively large number of guide teeth closely spaced is necessary in order to properly guide a gripper head travelling along with a flexible tape.
It is desirable that the gripper guiding is safe and positive along the entire weaving width, even when the weaving width is rather broad and even when high speed looms are involved.
Another possibility of guiding the gripper through the shed involves making the two gripper tapes substantially stiffer than the tapes that are guided by the guide teeth mentioned above. For this purpose it is known to use the reed beam sole as a sliding guide surface for the gripper head. The warp threads of the lower shed also run across the reed beam sole. Rearwardly, the gripper heads are also guided if desired by a guide piece connected to the respective gripper head and riding along the reed or reed blades. However, in this type of construction there is no guiding at all or just a minimal guiding in an upward direction and in the direction of the beat up motion.
The above described type of guiding has the disadvantage that the gripper head can be relatively easily brought out of its path by the vibrations of the loom. This problem becomes the more aggrevated the broader the loom is and the faster the loom operates. However, wide weaving widths and a fast operation are an economic necessity.
German Patent Publication 1,710,292 corresponding to U.S. Pat. No. 3,665,975 (Kokkinis), patented May 30, 1972 discloses control levers of a device for the positive controlling of the weft thread transfer from one gripper head to the other. These control levers reach through the warp threads of the lower shed. This type of arrangement is an improvement over the above mentioned construction, because irregularities in the fabrics, such as stripes or the like are avoided. This avoidance is accomplished in that the control levers are withdrawn out of the lower shed immediately after the transfer of the weft thread from one head to the other. That is, the withdrawal of the control levers takes place substantially before the actual beat up or weft binding takes place.
Further, one should take into account that a positive guiding of the gripper heads or of the tapes or rods carrying these heads is actually only necessary until the proper transfer of the weft thread from one head to the other has taken place without any collision of the two gripper heads. Thereafter, that is, during the reverse and outward motion of the grippers, it is no longer critical if there are small deviations from the ideal track of the grippers. German Patent Publication 1,710,292 mentions the possibility that the above control levers may perform, in addition to their controlling the gripper clamping lever, a function of centering or fixing the position of the gripper rod during the thread transfer or thread take over. However, a solution for such centering of the position of the gripper rods is not disclosed in this reference. German Patent Publication 3,901,549 (Gerhring), published May 23, 1990 further discloses a gripper with a guide element made of wood and attached to the gripper on its side facing the reed. The purpose of the wood piece is to provide a stabilizing guide along the reed blades for the gripper head as it carries the weft thread into the shed. As a result, the gripper head travels with its wooden guide element along the reed during its alternating back and forth movements into and out of the shed. This type of construction has several disadvantages. One disadvantage is seen in that the particular configuration and securing of the wooden guide element to the gripper is problematic. Another disadvantage is seen in that the individual reed blades of the slay are exposed to a continuous wear and tear by the friction with the wooden guide element.